The present invention relates to display devices. More particularly, the invention relates to a display device in which a stored visible image is formed on a display element by an optical signal.
One of the most popularly employed display devices is a film reader. The film reader has a screen in an optical image forming section, and an enlarged image is produced by the utilization of the variations in intensity of light diffused and passed through the screen. The conventional film reader is disadvantageous in that when the ambient light is high in brightness, or when the screen is observed at other than a direct angle, the contrast of the viewed image is low making it difficult to read the image on the screen.
Furthermore, although an enlarged image on the film reader has as a whole, a lower contrast than an ordinary document, it is nevertheless necessary for the film reader to project characters on an enlarged scale. In addition, since it is necessary to maintain the light source in the ON state the entire time the image is being read, the quantity of heat generated thereby is large. Accordingly, it is necessary to use heat resistant components in the construction of the reader and to provide some way to radiate heat. Moreover, the power consumption is large. Thus, the conventional film reader is not economical.
In order to eliminate the above-described drawbacks, a display device using a material such as liquid crystals having an electro-optical effect and a photosensitive material in combination has been proposed in the art.
Such a display device can be implemented as a liquid crystal panel formed by laminating with an insulating layer (or a dielectric mirror) a photosensitive material which has a spectral sensitivity corresponding to the wavelentgh of an image writing light source but not a spectral sensitivity corresponding to the wavelength of an image reading light source, and a liquid crystal.
However, the liquid crystal panel is disadvantageous in that the manufacturing cost thereof is high and the manufacturing yield is low because the manufacture of the liquid crystal panel requires a large number of manufacturing steps and a variety of materials which are difficult to handle. Also, the arrangement and operation of the driving circuit thereof are rather intricate.